Related Art
With diamond-coated hard materials comprising a substrate and diamond coated thereon, the diamond coating layer has only a low adhesion strength with respect to the substrate, such that the diamond coating layer tends to be peeled off from the substrate. Various techniques, such as exemplified hereinbelow, have hitherto been proposed in order to improve the adhesion strength of the diamond coating layer on the substrate.
In JP Patent KOKAI Publication No. 1-246361 (JP-A-246361/89) there is disclosed a sintered alloy having a specified coating film formed on a heated surface of a sintered alloy having a specified composition.
In JP Patent KOKAI Publication No. 4-231428 (JP-A-231428/92), there is disclosed a method for producing a machining tool comprising secondary sintering of a cemented carbide (superhard alloy) tool of a particular composition under specified conditions followed by chemical etching and ultrasonic grinding/polishing for forming a diamond coating layer.
In JP Patent KOKAI Nos. 4-263074 (JP-A-263074/92) and 4-263075 (JP-A-263075/92), there is disclosed a hard material comprising a substrate presenting specified surface irregularities and a diamond coating layer formed thereon.
Among the techniques preceding the above techniques, there are those described in JP KOKAI Patent Publication No. 54-87719 (JP-A-87719/79) and No. 58-126972 (JP-A-126972/83) corresponding to JP Patent KOKOKU Publication No. 62-7267.
In "Powder and Powder Metallurgy", vol. 29, No. 5, pages 159 to 163, there is report on the formation of a hard layer on the surface of a WC-.beta.-Co alloy, where .beta. is a WC-TiC solid solution. Thus it is stated therein that, if the alloy is heated at 1673K in N.sub.2 at 5.1 kPa (5.times.10.sup.-2 atmosphere), a hard layer presenting sharp surface irregularities is formed; such hard layer is formed at a N.sub.2 pressure of not lower than about 0.7 kPa (about 7.times.10.sup.-3 atmosphere); and that the higher the N.sub.2 pressure and the longer the heating time employed, the coarser become the grains (.beta.(N) grains) of the WC--TiC--TiN solid solution and the more outstanding become the surface irregularities, with formation of Co pools on the surface region.